RFR: 8371986: Remove the default value of InitialRAMPercentage

[Aleksey Shipilev]

On Thu, 27 Nov 2025 09:53:56 GMT, Joel Sikström <jsikstro at openjdk.org> wrote:

> Hello,
> 
> This RFE changes the default of `InitialRAMPercentage` to 0, effectively removing its default value. Please see the CSR for specific details on this change.
> 
> Changing the default value to 0 results in the behavior that the initial heap size (InitialHeapSize) is set to the minimum heap size (MinHeapSize). This is because of the following lines in `Arguments::set_heap_size()`, which takes the MAX value of MinHeapSize as well:
> 
> uint64_t initial_memory = (uint64_t)(((double)MaxRAM * InitialRAMPercentage) / 100);
> ...
> 
> reasonable_initial = MAX3(reasonable_initial, reasonable_minimum, MinHeapSize);
> reasonable_initial = MIN2(reasonable_initial, MaxHeapSize);
> 
> 
> This change improves startup performance for all GCs, but affects the time-to-peak performance in some out-of-the-box configurations for some GCs. This is mainly visible in ParallelGC.

Marked as reviewed by shade (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/28530#pullrequestreview-3515365711